I will reiterate what Kaldor is telling you guys, and give my thoughts on best practices and efficiency. I know efficiency isn't a high priority for a lot of you since you enjoy the process, but for many of us with limited time/a lot of brass to process it is very important.
1.) As stated a case gauge or using a comparator/caliper is a way to verify your equipment is calibrated properly to produce brass that will headspace correctly. A case gauge will also tell you if it needs to be trimmed as will dial calipers. This is a safety and quality control issue, that has to be adhered to in order to produce safe, reliable ammunition. Always verify the sizing die is adjusted properly with one of these two tools, just because the die manufacturer tells you to screw the die down all the way and back off 1/4 turn or whatever doesn't mean that will always work. The case gauge or comparator gives you a real number to verify off of, the die instructions are only a starting point.
2.) Kaldor points out, and he is 100% correct, that case gauging fired brass or using a comparator/caliper on fired brass tells you basically nothing of any value at all. Nothing. All you manage to accomplish by measuring fired brass is to waste time (to be brutally honest), and to verify that: yep it's been fired... theres carbon on it, a dent in the primer, and the brass is expanded, all surefire signs it went boom and got ejected out of a firearm. Your powers of deduction are keen!
So let's talk about this fired brass that you propose to process. Some of you are now irritated at me telling you you're wasting time case gauging this fired brass before you resize it in a properly set sizing die (that you set with your case gauge or comparator/caliper), you no doubt will point out that some of that brass really wouldn't fit in the gauge! It's all swollen at the web you say!
My response is: so what? I don't care how big the chamber it was fired in is, that doesn't matter. What matters is how well it resizes back to spec in your sizing die. This can usually be felt during the sizing process, every now and again it requires abnormal effort to size a case that is clean and lubed. More often than not if you yank that case after resizing you find it doesn't have a shoulder bumped back like other cases that resize normally and gauge just fine, this tough case doesn't gauge just fine. My suggestion is to throw it away. Obviously there is an issue with it, probably not once fired and it is now work hardened, who knows what other problems are lurking in the case web or primer pocket. So toss it. When you feel it takes abnormal force to run a piece of brass in your resizing die, always always always measure that case in your gauge with calipers, or comparator compared to other pieces, if it doesn't measure to spec toss it, even if it does best practice is to toss it. There is something wrong with that case, and it needs to go in the trash not in your rifle's chamber!
Notice that you discover problems with brass during actual resizing, not before. Many times that case you found to be larger than the others if you wasted time gauging it in fired condition prior to resizing, will resize just fine meaning you learned nothing of any value gauging it before resizing.
3.) Case gauges or comparators are great tools, but once dies are set up properly are only really used to verify proper adjustment before starting a batch and to periodically spot check brass to ensure adjustments are holding. With the exception of the occasional problem child brass that will have to be tossed after it fails gauging after resizing.
4.) Gauging every case prior to resizing is a waste of time, sorry but there it is. If you are processing a large batch of brass your best practices to maximize productivity mean handling individual cases as little as possible and letting your tools do the work. If you have good tools, and you know how to adjust them and check the adjustment your quality will be just as good or better than someone who laboriously fiddles with every case. Frankly this obsession with OCD levels of case prep is a distraction, and probably results in no actual improvement to the ammo. If your brass gets properly cleaned, resized with appropriate headspace, and trimmed to appropriate length it will work just fine once final loaded. The process is not rocket surgery, and no amount of futzing around with the brass outside of those basic steps will improve the function or accuracy of the ammunition. I have discovered this the hard way, and so have others who obsessed over things like run out etc. One fellow who posts on M4carbine.net vented his frustrations at how good Black Hills match ammo is, even though all of his runout gauges and other measurements suggested his hand loads should outperform the factory stuff; yet when sent downrange the results were the same. Not just for 3 shot anecdotal groups, but for multiple 10 shot groups, and long range shooting. So I'll reiterate, buy good tools and dies, adjust them properly for your application, use them correctly and process your brass and load your ammo. Then get out and shoot! Stop obsessing over minutia that don't mean anything, you gain nothing.
Good luck everyone!
